my Morley SLWV is powered by 12 to 18 volts DC out of what appears to be a small jack plug, tip positive. To do this using my PPP2+ I assume I would need a y-cable with 2 5.5x2.1mm barrel connectors on one end and a small jack plug on the other.

Are any such cables available on the market? If so, where?

Apart from making such cable myself, any other easy solutions?
I considered using two cable with battery snaps, but the way the battery compartment is placed on the bottom side, with a metal plate covering it makes that not a great solution.

The allowable voltage range can help here. For 12V operation you would use any one of outputs 1-4 with the corresponding DIP switch set away from Normal. Output 5 or 6 with the corresponding DIP switch set away from Normal would probably provide about 16V to this particular pedal. There is a Mini Plug Cable that comes with the Pedal Power 2+ that should work for this.
If you wanted to run specifically at 18V you could use the Mini Plug Cable together with our Voltage Doubler Adapter connected to any two outputs with the corresponding DIP switches set to Normal.